A tumor is an abnormal growth of tissue resulting from the uncontrolled, progressive multiplication of cells, serving no physiological function. A tumor may be malignant (cancerous) or benign. A malignant tumor is one that spreads cancerous cells to other parts of the body (metastasizes) through blood vessels or the lymphatic system. A benign tumor does not metastasize, but can still be life-threatening if it impinges on critical body structures such as nerves, blood vessels and organs (especially the brain).
A non-invasive method for tumor treatment is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source is changed, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low. The term radiotherapy refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centi-Gray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment) and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted by the magnitude of the radiation
One problem encountered in external beam radiation treatment is that pathological anatomies (e.g., a tumor) may move during treatment, which decreases accurate target localization (i.e., accurate tracking of the position of the target). Most notably, soft tissue targets tend to move with patient breathing during radiation treatment delivery sessions. Respiratory motion can move a tumor in the chest or abdomen, for example, by more than 3 centimeters (cm). In the presence of such respiratory motion, for example, it is difficult to achieve the goal of precisely and accurately delivering the radiation dose to the target, while avoiding surrounding healthy tissue. In external beam radiation treatment, accurate delivery of the radiation beams to the pathological anatomy being treated can be critical, in order to achieve the radiation dose distribution that was computed during the treatment planning stage.
One conventional solution for addressing the problem of tumor motion due to respiration is the use of gating techniques. Gating techniques dose not directly compensate for breathing motion, in that the radiation beam is not moved while it is being directed in the patient. Rather, the radiation beam is turned off when the tumor is thought to have moved from its reference position. However, a disadvantage of using a gating technique is that it significantly increases the amount to time required for delivering the radiation treatment. Another disadvantage is such an approach may result in inaccurate treatment of the tumor due to the assumptions made in tumor position.
One conventional solution for tracking motion of a target utilizes external markers (e.g., infrared emitters) placed on the outside of a patient (e.g., on the skin). The external markers are tracked automatically using an optical (e.g., infrared) tracking system. However, external markers cannot adequately reflect internal displacements caused by breathing motion. Large external patient motion may occur together with very small internal motion. For example, the internal target may move much slower than the skin surface.
Another conventional solution for tracking motion of a target involves the use of implanted fiducials. Typically, radiopaque fiducial markers (e.g., gold seeds or stainless steel screws) are implanted in close proximity to, or within, a target organ prior to treatment and used as reference points during treatment delivery. Stereo x-ray imaging is used during treatment to compute the precise spatial location of these fiducial markers (e.g., once every 10 seconds). However, internal markers alone may not be sufficient for accurate tracking. Yet another conventional solution combines the tracking of internal fiducial markers with the tracking of external markers in which x-ray imaging of the internal fiducial markers is synchronized with the optical tracking of the external markers. However, such a combined tracking approach still has the disadvantage of requiring the tracking of internal fiducial markers.
The tracking of internal fiducial markers can be difficult for the patient, because high accuracy tends to be achieved by using bone-implanted fiducial markers. The implanting of fiducial markers in bone requires a difficult and painful invasive procedure, especially for the C-spine, which may frequently lead to clinical complications. In addition, tracking bone-implanted fiducial markers may still may not provide accurate results for movement or deformation of soft tissue targets. Moreover, whether the fiducial marker is implanted in the bone or injected through a biopsy needle into soft tissue in the vicinity of the target area under computerized tomography (CT) monitoring, the patient must still undergo such invasive procedures before radiation treatment.
A conventional technique that tracks the motion of a tumor without the use of implanted fiducial markers is described in A. Schweikard, H Shiomi, J. Adler, Respiration Tracking in Radiosurgery Without Fiducials, Int J Medical Robotics and Computer Assisted Surgery, January 2005, 19-27. The described fiducial-less tracking technique use deformation algorithms on CT data sets, combined with registration of digitally reconstructed radiographs (DRR) and intra-treatment X-ray images of nearby bony landmarks (where the tumor itself may not be visible in the x-ray image in most cases) to obtain intermittent information on the tumor location. This target location information is then combined with conventional correlation techniques to achieve real-time tracking.
One disadvantage with all the above described conventional methods is that, with the exception of external marker tracking, they require the repeated exposure of the patient to non-therapeutic radiation from the intra-treatment x-rays that are taken to obtain intermittent information on the fiducial or target location.